1. Field of the Invention
The present invention relates to scanning electron beam apparatus and methods for inspection or defect review of substrates, such as semiconductor wafers and masks.
2. Description of the Background Art
Scanning electron beam systems are utilized for the inspection and defect review of semiconductor substrates and for other purposes. A conventional scanning electron beam system utilizes a straight optical axis down an electron-optical column. The signal electrons are generally detected with either an annular detector, a combination of a Wien filter and an off-axis detector, or an Everhart-Thornley detector (secondary electrons and no extraction field only).
With a straight optical axis, the detection electron-optics overlaps with the illumination electron-optics. This overlap often disadvantageously limits the detection functionality, efficiency and flexibility.
For example, with an annular detector, it is difficult to detect all the signal electrons under all imaging conditions, where the imaging conditions may include various landing energies, extraction field strengths, and Wehnelt electrode biases. This difficulty reduces the detection efficiency and limits throughput of an inspection system.
As another example, a Wien filter used to deflect the secondary electrons may induce aberrations of the primary beam. Such aberrations reduce the system resolution. In addition, the beam deflection tends to increase the region across which the signal electron move in the detector plane. This larger region in the detector plane results in a need for a larger area detector which limits the detection speed.
In addition, with a straight optical axis, it is often difficult to discriminate the signal electrons based on their energy. In other words, it is often difficult to implement an energy filter with the conventional geometry.